A conventional hot runner system for the injection molding of “plastics” (thermoplastic polymers) utilizes an injection nozzle or an array of such nozzles to force or “inject” molten resin (“melt”) into a void defined between core and cavity parts of a mold. Each nozzle is made up of a nozzle body or housing and a nozzle tip secured to a “tip end” of the housing either by direct threaded engagement or through the use of a “nozzle cap”. A conventional nozzle cap is a ring which fits over the nozzle tip, engages a flange at the base of the nozzle tip and threadedly engages the tip end of the nozzle housing.
The housing and the tip have generally axially extending passages therethrough which register with one another to define a melt passage along which the melt flows during injection. The tip may have one or more outlets at its end depending on the flow pattern required for the mold arrangement being used. In some cases the tip would have a single outlet axially aligned with the melt passage which may be blocked and opened by a valve pin extending along the melt passage and through a base of the nozzle housing. Alternatively, melt flow at the tip may be controlled through “sprue gating” according to which solidification of a portion of the melt in the tip is used to interrupt melt flow. The nozzle tip may be received in a “gate insert”. The gate insert forms the gate portion of a mold, which is that portion through which melt enters the mold.
The resin must be maintained in its molten state as melt until the mold is filled. As a gate insert and its surrounding mold part represent a large heat sink, and the nozzle tip is traditionally of metal (which conducts heat well), prior practice has been to avoid direct contact between the nozzle tip and the gate insert. This has been achieved in various ways in the past. One way to avoid direct contact between the nozzle tip and the get insert is to provide a “gap” or “void” between the outer end of the nozzle tip and the gate insert. Initially the gate would be filled with air but later would fill with resin which seeps in during molding. A disadvantage to such an arrangement occurs during changeover from resin of a first colour to resin of another colour. Trapped resin continues to “bleed” into the new resin producing unacceptable streaking and requiring a lengthy changeover and many wasted parts before changeover is fully established.
Another solution to the problem of bleeding is to fill the void between the gate insert and the nozzle tip with an insert sometimes called a “gate well insulator”. The insulator is a machined insert of an insulating material such as a ceramic such as for example sold under the “Vespel” brand, which occupies the void in which resin would otherwise collect. This approach has disadvantages which include an expensive base material, difficulty in matching the insulator to the gate inserts and the tips, cracking of the insulator after a short period of use and sticking of the insulators to the gate insert when the mold is opened for cleaning.
Another solution has been proposed in my earlier U.S. Pat. No. 6,709,262 B2 pursuant to which a nozzle cap is provided which is machined to very close tolerances to provide a gap between the nozzle tip and the gate insert large enough to avoid conductive heat transfer yet small enough to prevent melt flow. Heat loss may still prove problematic in this approach in some applications.